1. Field of the Invention
The present invention relates to a photocurable composition and a curing process therefor. In particular, the present invention relates to a photocurable composition which can be cured not only on the curing surface but also to the interior of the cured product with notably minimized coloration of the cured product resulting from the polymerization initiator, by irradiation with ultraviolet light in the range of 200-400 nm and visible light in the range of 400-740 nm, even when using materials which exhibit poor light transmittance as a result of containing pigments and coloring dyes with strong hiding powers or having large thicknesses, as well as to a process for curing the photocurable material.
2. Description of the Related Art
Photopolymerization has been conventionally employed for a wide range of multifaceted uses, including curing and printing of coatings, resin relief-printing, printed board construction, resist and photo masking, black-and-white and color transfer coloring sheets and coloring sheet preparation, etc., and the recent focus on environmental issues, energy conservation and labor-saving means to counter increasing labor costs has stimulated active development with an emphasis on polymerization at normal temperature, rapid curing and the ability to be used without a solvent, which are features of photopolymerization.
In the area of printing ink, research has been progressing an ultraviolet photocuring inks for the purpose of elimination of solvents, improving productivity through achieving more rapid curing, or provision of superior physical characteristics for cured coatings (For example, Japanese Unexamined Patent Publications No. 1-229084 and No. 2-22370). In Japanese Unexamined Patent Publication No. 2-22370 there is disclosed an ultraviolet photocuring ink with an excellent-curing rate and coating hardness, which is characterized by containing a poly(meth)acrylic acid ester and an ultraviolet photopolymerization initiator.
Also, while research in the area of coating paints continues with respect to rapid curing and curing of colored coatings (for example, Japanese Unexamined Patent Publications No. 52-135341, No. 52-152436, No. 2-296875 and No. 3-6265), curing with ultraviolet light results in rapid curing of polymerizable monomers when cationic polymerization or radical polymerization is performed with irradiation of ultraviolet light of 200-400 nm, and therefore penetration of ultraviolet light has been a problem with colored coatings containing pigments or dyes with strong hiding powers.
For example, when a pigment or a coloring dye with a strong hiding power is added to give a colored coating with the aforementioned superior hiding power, the light transmittance is impaired by absorption or reflection of light by the pigment or coloring dye, resulting in a problem in which the coating surface cures but the interior of the coating fails to sufficiently harden. Also, even in transparent compositions containing no pigments or coloring dyes, if the curing composition thickness is increased or the curing composition contains a material with poor ultraviolet transmittance, the same problem occurs in which the interior of the curing composition fails to sufficiently harden.
In order to overcome this problem, an attempt has been made in Japanese Unexamined Patent Publication No. 6-75374 to increase the interior curability of photocurable compositions containing pigments, by adding a near-infrared radical polymerization initiator comprising a near-infrared light absorbing cationic dye and an organic boron salt sensitizer to an ultraviolet radical polymerization initiator. However, this composition has had a disadvantage in that the ultraviolet decomposition product of the near-infrared light absorbing cationic dye has caused coloration of the material after photocuring.